• Divides cytoplasm
• Animal Cells
– Microfilaments constrict/pinch the cytoplasm
• Plant Cells
– Cell plate forms between two daughter nuclei
– Cell walls then form on either side of cell plate

• Interphase
• Prophase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis
MITOSIS

”gametes”, with half the number of chromosomes, are produced.
During Meiosis diploid cells are reduced to haploid cells
Diploid (2n)

Haploid (n)
If Meiosis did not occur the chromosome number in each new generation would double…. The offspring would die.

Interphase
Meiosis I
Meiosis II
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II

2n
4n
Interphase/
DNA replicates
Meiosis II
Meiosis I
2n
2n n n n n

-
Homologous chromosomes sister chromatids
Tetrad sister chromatids

( because a homologous pair consists of 4 chromatids it is called a “Tetrad”) eye color locus eye color locus hair color locus
Paternal Maternal hair color locus

• Interphase –replication
• Prophase I - homologous chromosomes pair
– Crossing over! creates variation (diversity) nonsister chromatids Tetrad chiasmata: site of crossing over

• Metaphase I
– Homologous pair line up at center (tetrad)
– INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair is random
2. **Variation** produces 2 n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 and 2 23 = 6,000,000

• Anaphase I
– Homologous pairs move to opposite sides of cell
• Telophase I/Cytokinesis
– Cell divides

1. How many cells are at the end of Meiosis 1?
2. How many chromosomes are in each cell at the end of meiosis 1?

2n
4n
Interphase/
DNA replicates
Meiosis II
Meiosis I
2n
2n n n n n

• Prophase II - no DNA replication!!!
• Metaphase II
• Anaphase II
• Telophase II/Cytokinesis

1. How many cells are at the end of Meiosis 2?
2. How many chromosomes are in each cell at the end of meiosis 2?

Two cell divisions
Half the original chromosomes
Homologous
Chromosomes pair up
Cytokinesis
Four daughter cells
46 Chromosomes at end
Meiosis Mitosis

1. Number of Cells
– Mitosis creates 2 identical cells
– Meiosis creates 4 individual cells
2. Number of chromosomes
– Mitosis creates cells with 46 chromosomes (2n – diploid)
– Meiosis creates cells with 23 chromosomes (n – haploid)
3. Locations
– Mitosis – somatic (“normal”) cells
– Meiosis – gametes (sex) cells

• The Key Difference Between Mitosis and
Meiosis is the Way Chromosomes Uniquely
Pair and Align in Meiosis
Mitosis The first (and distinguishing) division of meiosis

1. If a cell has 40 chromosomes, how many are in each cell at the end of meiosis 1?
2. Meiosis 2?
3. Mitosis?
4. T/F: Mitosis creates 2 different cells
5. T/F: A difference between meiosis & mitosis is that meiosis occurs in gametes and mitosis occurs in somatic cells.

Meiosis Mitosis
Chromosome Number: Reduced by half (haploid) Remains the same (diploid)
Pairing of Homologues: Yes No
Function: sexual reproduction
Cellular (asexual) Reproduction; general growth and repair of the body
#of Divisions:
#of Cells produced:
Creates:
Crossing Over:
Genetically:
2
4
Sex cells (Gametes)
Yes different
1
2
Somatic Cells
No identical

• Genetic Variation
• Random distribution of chromosomes
• Chromosomes are passed to offspring independently of each other

http://wps.prenhall.com/esm_freeman_biosci_1/7/1948/498784.cw/index.html

• The fusion of a sperm and egg to form a zygote .
• A zygote is a fertilized egg n=23 egg sperm n=23
2n=46 zygote

• Asexual reproduction involves a single parent.
• genetically identical offspring
• All prokaryotes and some eukaryotes
• can be very rapid
– allows them to crowd out other organisms that reproduce more slowly

Binary fission - a parent cell splits into two identical daughter cells (E. Coli)
Fragmentation - a parent organism breaks into pieces and each piece develops into a new organism (Starfish – sailors thought they were killing them… not so much  )
Budding - a parent cell forms a bubble-like bud. The bud stays attached to the parent cell while it grows and develops. When the bud is fully developed, it breaks away from the parent cell and forms a new organism.
JELLYFISH don’t rely solely on budding to reproduce – 1 st release a sperm & egg into the H20. Zygote grows into a basic organism called a polyp, which lives attached to the base of a rock. The polyp then grows, before releasing a genetically identical bud of itself, that grows into an adult jellyfish.

• involves two parents
• parents produce gametes
• Gametes are haploid cells
• two gametes unite is called fertilization
• The fertilized cell is a zygote .
• A zygote is diploid cell

Asexual
• Organisms inherit all chromosomes from single parent
– Genetically identical
• Little energy
• No mate needed
• Good for stable env’t
• Bacteria
Sexual
• Chromosomes from 2 parents
• Genetic variation
– Good for changing env’t
• Most animals/organisms
Why reproduce sexually?
Greater chance of beneficial mutations
• Crossing over, random assortment

Meiosis Mitosis
1. Two cell divisions
2. Occurs in gametes
3. Half the original chromosomes
4. Homologous Chromosomes pair up
5. Creates identical cells
6. Four daughter cells
7. Same #of chromosomes at end

2. Metaphase I
1. Prophase I
3. Anaphase I
4. Telophase/Cytokinesis I

1. Prophase II
Cells from Meiosis I
2. Metaphase II
3. Anaphase II
4. Telophase II
/Cytokinesis II